Many metals of industrial importance undergo a chemical change when exposed to the action of air and water. This change often results in the production of spots of differently colored material scattered over the surface of the metal, e.g., reddish brown rust on iron and steel, white specks on aluminum and blue or green layers on copper. The change not only spoils the appearance of the metal but sometimes penetrates so deeply into the metal that its mechanical properties are seriously impaired. This deterioration of a substance or its properties because of a reaction with the environment is commonly termed corrosion.
Corrosion of metals is essentially an oxidative process whose exact mechanism is often quite complicated. Simply stated, the rusting of iron, for example, requires three elements: water, oxygen and some impurity often provided by the carbon dioxide present in the air. The action of the impurity is presumably to provide a feebly conducting solution with the water and so set up an electrolytic effect. Iron salts are formed initially but are subsequently oxidized to red-brown rust whose chemical composition is Fe.sub.2 O.sub.3.H.sub.2 O. Similar processes occur with other metals.
It follows, of course, that corrosion may be prevented, or at least retarded, by retardation of the electrolytic effect or by preventing contact of the metal surface with water and air. Preparations containing sodium dichromate or arsenic salts are "inhibitors" and are effective in preventing corrosion because they readily adsorb to the metallic surface and retard the flow of current thus stifling the electrolytic effect.
Alternatively, organic coatings may be applied as paints or protective sheaths which function to seal out water and air from the metal surface. Examples of some such corrosion inhibiting coatings are shown in U.S. Pat. Nos. 2,430,846 to Morgan, 2,471,638 to McCarthy and 2,995,532 to Cantrell.